This invention relates to a sheet discharging device for use in a printer, copying machine, or like image forming apparatus.
Normally, a toner image formed by an imaging assembly including a photosensitive member or the like is transferred to a sheet by a transfer device in an image forming apparatus. The sheet bearing the toner image is discharged after having the toner image fixed thereto by a fixing device.
The fixing device normally adopts a heating roller fixing method so as to carry out a fixing operation safely at a high speed, and thus includes a heating roller and a pressing roller. The surface of the heating roller is formed of, for example, Teflon rubber, The pressing roller is pressed against the heating roller at a specified pressure. The sheet having the toner image transferred thereto is caused to pass between the heating roller and the pressing roller, and thereby the toner image is fixed onto the sheet due to the heat given from the heating roller and the pressure given from the pressing roller.
The sheet having the toner image fixed thereto is discharged by a discharging device arranged downstream from the fixing device. The discharging device includes a pair of a drive roller and a driven roller. The discharging device is arranged so that a contact point of the drive roller and the driven roller is positioned on such a transport line as to satisfy the following conditions. Specifically, the transport line is a tangent line on a point of a circumferential surface of the heating roller which intersects with a straight line connecting centers of the heating roller and pressing rollers (Japanese Unexamined Patent Publication No. 3-72389). FIG. 5 shows a construction of this conventional device. As shown in this figure, the drive roller 104 and the driven roller 105 of the discharging device 103 are arranged such that a contact point Z thereof is positioned on a tangent line Y at a point at the circumferential surface of the heating roller 101 which intersects with a straight line X connecting the centers of the heating roller 101 and the pressing roller 102 of the fixing device 100.
The reason why the discharging device 103 is arranged at the above position is considered as follows. The sheet passing through the fixing device 100 is transported uniformly by the heating roller 101 and the pressing roller 102 without bending toward either the heat roller 101 or pressing roller 102. In order for the discharging device 103 to nip a leading edge of the sheet transported from the fixing device 100, it is considered good to arranged the drive roller 104 and the driven roller 105 at the aforementioned position.
However, since the surface of the pressing roller 102 is softer than that of the heating roller 101 as described above, the sheet passing through the fixing device 100 is curled toward the heating roller 101 as shown in FIG. 6. Further in the fixing device 100, the toner depositing on the sheets by an electrostatic force or the like is heated and pressurized to be fused and deformed, thereby being fixed onto the sheet. Accordingly, the sheet tends to roll around the heating roller 101 due to the fused toner. Thus, the sheet is in reality transported while being curled toward the heating roller 101 as indicated by an arrow in FIG. 5. Therefore, even if the leading edge of the sheet is nipped and discharged properly by the discharging device 103, the curled state of the sheet inadvertently generated in the fixing device 100 is still found since no force is applied to straighten the curled sheet in the discharging device 103.